The present invention relates to the field of thermosetting polymers and polymer composites with structural function.
These materials are used in many different applications, including transport vehicles (aircraft, spacecraft, boats, ships, cars, etc.), electronic components (for professional, civil, industrial and hobby use), sports articles, etc. Moreover, in service, they are subject to damage attributable to the formation of microcracks that are produced in the structure under the action of various kinds of stresses, for example: a) mechanical vibrations or various types of mechanical stresses, b) sudden temperature changes, c) irradiation by high-energy radiation causing direct or indirect rupture of chemical bonds (UV light, γ rays, etc.), d) intentional or inadvertent contact with chemical substances that adversely affect the structure, e) various factors which in combination can contribute to compromising the integrity of the structure.
To reduce the damage caused by the various stresses and thus increase the useful life, safety and reliability of these materials, various systems have been proposed and developed for making them able to initiate self-healing processes that are activated by the actual onset of the microcracks, as stated for example in patent documents U.S. Pat. No. 6,518,330 and WO-2009/113 025. As described in those documents, powders of a catalyst of ring opening metathesis reaction, and microcapsules containing a reactive monomer that can polymerize following a ring opening metathesis reaction and then undergo crosslinking, are dispersed in the matrix of the polymer material. Therefore, when a crack that forms in said material reaches a microcapsule, it breaks it, causing the monomer to be released. The latter, coming in contact with the catalyst, is polymerized and then crosslinked, thus stopping the crack and restoring the structural continuity of the matrix. A field of choice for the use of these materials is that of structural aeronautical components.
For preparation of the matrix of these composite materials, a precursor of the thermosetting polymer is mixed with a curing agent and is reacted at elevated temperatures, to obtain the matrix of thermosetting polymer. These mixtures also already contain the microcapsules containing the reactive monomer and the solid particles of metathesis catalyst, to remain embedded in the matrix formed so as to provide, if the need subsequently arises when the material is in use, the self-healing function mentioned above.
According to the prior art, the catalyst for the metathesis reaction is embedded in the precursors of the polymer matrix in the form of solid particles, i.e. powders with varying morphology and crystallographic modifications (cf. “Supporting Information”, G. O. Wilson, M. M. Caruso, N. T. Reimer, S. R. White, N. R. Sottos, J. S. Moore. Chem. Mater., 2008, 20, 3288-3297).
In practice, the effective concentration of the catalyst depends on the availability of the aforesaid particles at the level of the fracture and on the rate of dissolution of the catalyst in the reactive monomer within the polymer matrix. Even with high concentrations of particles of catalyst exposed at the level of the fracture, the effective concentration of the catalyst could be relatively low because of limited rates of dissolution of the catalyst. The rate of dissolution of the catalyst depends not only on the chemical nature of the various components, but also on morphological and structural characteristics of the catalyst, for example the local availability, dimensions and crystallographic modification of the particles.
Overall, therefore, it has been found in practice that the presence of the catalyst in the form of crystalline powders has some critical aspects relating to the uniform availability of the catalyst in all the zones in which a microcrack can potentially develop, compromising the effectiveness of the self-healing process.